1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter referred to as TFTs). More particularly, the invention relates to an electronic device as represented by, for example, a liquid crystal display and to an electric appliance using such an electronic device as a display unit. In this specification, the semiconductor device refers to devices in general that work by utilizing the semiconductor characteristics. Therefore, electronic devices, semiconductor circuits and electric appliances are all semiconductor devices.
2. Description of the Related Art
Thin-film transistors (TFTs) can be formed on a transparent substrate and, hence, development has been positively forwarded for applying them to the active matrix-type liquid crystal display (hereinafter referred to as AM-LCD). The TFT using a crystalline semiconductor film (typically, a polysilicon film) exhibits a high mobility, and makes it possible to realize a highly precise image display by integrating functional circuits on the same board.
The AM-LCD basically comprises a pixel unit for displaying a picture, a gate drive circuit for driving TFTs of the pixels arranged in the pixel unit, and a source drive circuit (or a data drive circuit) for sending pixel signals to the TFTs, that are formed on the same substrate. In this specification, the gate drive circuit and the source drive circuit are all referred to as drive circuit units.
In recent years, there has been proposed a system-on-panel in which signal processing circuits such as signal-dividing circuit and Y-correction circuit, too, are formed on the same substrate in addition to the pixel unit and the drive circuit unit.
However, the pixel unit and the drive circuit unit require different circuit performances, and it is difficult to satisfy all circuit specifications by the TFTs of the same structure. That is, the structure of the TFT has not yet been established to simultaneously satisfy the TFTs (hereinafter referred to as pixel TFTs) that constitute the drive circuit unit such as shift register circuit which gives importance on the high-speed operation and that also constitute the pixel unit which gives importance on the high breakdown voltage characteristics.
The present applicant therefore has filed an application covering a constitution in which the gate insulating film has a different thickness depending upon the TFT's (hereinafter referred to as drive TFTs) forming the drive circuit unit and the pixel TFTs (Japanese Patent Laid-Open No. 10-056184). Concretely speaking, in the above application, the thickness of the gate insulating film of the drive TFTs is decreased to be smaller than the thickness of the gate insulating film of the pixel TFTs.
In recent years, each pixel in the pixel unit has been formed having an area as very small as about 18 μm×18 μm to realize a picture of as highly fine as XGA (eXtended Graphics Array, which is 1024×768 pixels) on a liquid crystal panel having a diagonal of 0.9 inch. It is expected that the size of the pixels will be further reduced in the future.
The greatest problem that stems from a reduction in the size of the pixels is a decrease in the numerical aperture in the transmission-type liquid crystal display. That is, the effective area for displaying the picture decreases, and the brightness decreases. If it is attempted to increase the numerical aperture, a countermeasure must be taken, such as decreasing the area occupied by the TFTs or decreasing the area occupied by the storage capacitors.
Thus, performance required for the TFTs and occupied areas are placed under very severe conditions accompanying a reduction in the size of the pixels and, besides, areas occupied by the storage capacitors are placed under severe conditions, too, making it very difficult to design the structure of the pixels.